Electric thrusters are known in the art. Unlike chemical propulsion, where the specific impulse is limited by the energy available when molecules combine, in electric propulsion energy is added from an external source. In principle, therefore, the specific impulse can be as large as desired. In practice, of course, the specific impulse is limited by the particular implementation used. Since thrust will decrease as the specific impulse increases for a given power, a tradeoff must be made for a particular mission between propellant usage and mission time. High specific impulse leads to low propellant usage.
The tradeoff between electric propulsion and chemical propulsion is high thrust, low specific impulse for chemical and low thrust, high specific impulse for electric. Electric thrusters cannot be used for launch because the thrust is too low. Electric thrusters can only be used in the vacuum of outer space.
The are three main types of electric thrusters: electrothermal, electromagnetic, and electrostatic.
Electrothermal thrusters are similar to standard chemical rocket engines. Electrical energy is added to the working gas, but the gas is expanded through a converging-diverging nozzle to achieve high exhaust speeds just as in chemical rockets. Some examples are the resistojet, the arc jet, and microwave heated thrusters. In the resistojet, gases already hot from burning are further heated electrically. The arc jet uses an electrical arc to create very high temperatures. More recently, microwaves have been proposed to do the heating of the gas in thrusters which are otherwise like arcjets. As a class, electrothermal thrusters are probably the most mature electric propulsion technology, although the individual thruster with the most operational experience is the Teflon ablative type. Resistojets have been used for many years, and arcjets have also been used over the past few years in operational, commercial communications satellites. Compared to other electric thrusters, electrothermal devices have higher thrusts, but lower specific impulse, in the range of 500-1000 seconds. They share with chemical rockets an optimization when the molecular weight of the exhaust gas is low, unlike other electric thrusters.
There are a variety of electromagnetic thruster configurations, but all depend on generating a thrust by accelerating particles in a direction perpendicular to both the current in the plasma and the magnetic field. The pulsed plasma microthruster (PPT) utilizes a spark discharge across a block of Teflon to create plasma which is accelerated outward by induced azimuthal current interacting with a radial magnetic field. In a Hall thruster an axial electric field provided in a radial magnetic field creates an azimuthal Hall current which accelerates plasma axially producing thrust. In the self-field magnetoplasmadynamic (MPD) thruster, the current flow creates its own magnetic field in which the j.times.B force accelerates the plasma flow radially and axially. This can only occur if the current and hence the power are high, necessitating pulsed operation at lower average powers. Interestingly, the self-field MPD thruster is similar to the electrothermal arcjet. The MPD regime is reached when the mass flow is reduced.
In general, electromagnetic thrusters have much higher specific impulse than electrothermal thrusters do. They are more compact than electrostatic ion thrusters are because a charge neutral plasma does not have a space charge limitation on density. Problems include electrode erosion and general complexity of flow and current fields which make them somewhat difficult to predict. The PPT thruster is mature and simple, but harder to scale up to large powers.
Electrostatic ion thrusters use a set of grids to accelerate charged ions. Electrons are also expelled separately to maintain charge neutrality and prevent a charge buildup which could shut off the ion beam. Heavy gases are used; mercury was used in the initial versions and xenon is used today. This reduces ionization losses as a fraction of total energy. Ionization losses are approximately the same for most gases, whereas for a given exhaust velocity the energy added per ion is greater for heavier gases.
In electrostatic thrusters the beam consists of ions only and repulsion between particles limits the maximum density to relatively low levels. The electrostatic thruster offers significantly lower thrust than conventional RF plasma thrusters.
The prior use of RF plasma thrusters has suffered from poor efficiency due primarily to power loss through a hot electron population created by electron cyclotron resonance (ECR) heating of the plasma. The use of ECR has several disadvantages. The major disadvantage is the creation of a hot electron population that robs the thruster of power, leading to low efficiency. Other disadvantages include the ECR heating requires higher frequencies for a given set of plasma parameters than other RF heating schemes. Higher frequency RF sources are generally more expensive and less efficient. Additionally, the frequency and magnetic field must be precisely matched. Plasma densities are usually limited to less than the cutoff density for a given, frequency.
An ECR generated plasma contains populations of electrons with different temperatures. A hot population forms because of "runaway". Electron drag and collision cross section decrease as electron energy increases. Once an electron reaches a critical energy, it "runs away" because the drag can no longer balance the RF energy absorbed. An electron in resonance with the RF field essentially sees a continuous DC field, as the field rotates at the same rate as the electron as it spirals around the magnetic field line. The electron energy increases until some other process limits the energy. The ultimate limit for magnetic mirror machines occurs when the electron energy is high enough that the adiabatic invariant is no longer conserved and electrons are no longer trapped in the mirror. Hot electrons are generally produced by using twice the fundamental frequency, which is more effective at heating hotter particles.
In most of these devices there are particular reasons for producing the hot electron population. Hot electrons take almost all their energy with them when they are lost because their energies are so much greater than the plasma potential. They also tend to absorb more RF power than colder electrons. For a thruster, all the power entering the warm or hot electrons is simply wasted. All ECR plasmas on which there were diagnostics capable of observing hot electrons have shown split electron populations. Power balance calculations show that about 1% of the RF power was going into the cold plasma in the ECR plasma and somewhere between 50% and 100% of the RF went into the cold plasma in the lower hybrid generated plasma. It would be desirable to have a RF plasma thruster which has high efficiency, utilizes lower frequency RF sources than ECR heating, and does not suffer from hot electron runaway.
RF plasma thrusters are also simpler than electromagnetic thrusters, which generally have currents perpendicular to the magnetic field, which crossed with the magnetic field produces the thrust. The currents produce their own magnetic field, which in the worst case can go unstable. In any case, the current produces its own magnetic field which interacts with the imposed magnetic field. This makes scaling of devices to different sizes difficult. By contrast, in the RF plasma thruster each flux tube is like any other. The rapid axial transport of particles compared to radial movements means there is little interaction between flux tubes, so scaling up (or down) in size is very predictable.
It would be desirable to have a RF plasma thruster that does not suffer from poor efficiency while providing a high specific impulse, high power density and is adaptable to many different applications.